The prominent and rapidly expanding role of mass spectrometry (MS) in the physical and biological sciences can be attributed in part to the versatility afforded by the growing catalog of available ionization methods. Many ionization techniques of increasing importance operate at elevated or atmospheric pressure, including electrospray ionization (ESI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), and desorption electro-spray ionization (DESI). To achieve the maximum possible sensitivity, ions created at atmospheric or higher pressures must be transmitted into the mass spectrometer with high efficiency through a narrow, conductance limiting aperture.
Ion transfer from the ambient environment into a mass spectrometer is a problem associated with ambient ionization techniques. Generally, in ambient ionization, ions are generated at atmospheric pressure and subsequently transferred into a mass spectrometer that operates under vacuum, i.e., having separate differentially pumped vacuum chambers that ions pass through prior to reaching the high vacuum region of the mass analyzer. To maintain the vacuum, a mass spectrometer is coupled to continuously operating pumps, which consume a large amount of power. Accordingly, an inlet of a mass spectrometer is generally kept as small as possible to minimize vacuum pumping requirements on the mass spectrometer. Having a small inlet decreases ion transfer efficiency into the mass spectrometer, limiting system sensitivity by preventing a certain number of ions from ever entering the mass spectrometer. The ion transfer efficiency (as well as the total ion flux) can be increased by increasing the size of the inlet. However, increasing the inlet size makes it more difficult to maintain the mass spectrometer under vacuum, increasing the stress and power requirements on the pumps of the system.